The present invention relates to electric cables and particularly the invention relates to semiconducting layers of electric cables, preferably to cross-linked, semiconducting layers of electric cables, and more preferably to cross-linked, inner and non-strippable outer semiconducting layers of electric cables.
Electric cables and particularly electric power cables for medium and high voltages are composed of a plurality of polymer layers extruded around the electric conductor. The electric conductor is usually coated first with an inner semiconducting layer followed by an insulating layer, then an outer semiconducting layer. To these layers further layers may be added, such as a water barrier layer and a sheath layer.
The insulating layer and the semiconducting layers normally consist of ethylene homo- and/or copolymers which preferably are cross-linked. LDPE (low density polyethylene, i.e. polyethylene prepared by radical polymerisation at a high pressure) cross-linked by adding peroxide, for instance dicumyl peroxide, in connection with the extrusion of the cable, is today the predominant cable insulating material. The inner semiconducting layer normally comprises an ethylene copolymer, such as an ethylene-vinyl acetate copolymer (EVA). The composition of the outer semiconducting layer differs depending on whether it has to be strippable or not. Normally a strippable semiconducting layer comprises an ethylene copolymer, such as an EVA together with an acrylonitrile-butadiene rubber (NBR) and sufficient carbon black to make the composition semiconducting. A non-strippable (bonded), outer semiconducting layer may comprise EVA, EEA or EBA together with an amount of carbon black sufficient to make the composition semiconducting.
As an example of non-strippable type semiconducting compositions, mention may be made of EP 0 057 604 which discloses, in its examples, three semiconductive compositions, where semiconductive composition A contained 59.25% by weight of an EVA containing 20% by weight of vinyl acetate (VA) comonomer, 40.00% by weight of acetylene black, 0.2% by weight of antioxidant and 0.55% by weight of dicumyl peroxide (DCP) cross-linking agent, composition B contained 83.8% by weight of an EVA containing 18% by weight of VA comonomer, 15% by weight of Ketchen black, 0.2% by weight of antioxidant and 1.0% by weight of DCP and composition C contained 61.2% by weight of EVA containing 20% by weight of VA comonomer, 38% by weight of furnace black, 0.2% by weight of antioxidant and 0.6% by weight of DCP.
As an example of prior art strippable semi-conducting compositions for electric cables, mention may be made of U.S. Pat. No. 4,286,023 which discloses a polymer composition for electric cables comprising (A) an ethylene copolymer selected from the group consisting of ethylene-alkyl acrylate copolymers containing about 15-45% by weight of alkyl acrylate, said alkyl acrylate being selected from the group consisting of C1-C8 alkyl esters of (meth)-acrylic acid, such as methyl acrylate, ethyl acrylate, methyl methacrylate, butyl acrylate, 2-ethylhexyl acrylate and the like, and ethylene-vinyl acetate copolymers containing about 15-45% by weight of vinyl acetate, (B) a butadiene-acrylonitrile copolymer (nitrile rubber) containing about 10-50% by weight of acrylonitrile, (C) conductive carbon black, and (D) a peroxide cross-linking agent, wherein the weight ratio A:B=1:9 to 9:1; C:(A+B)=0.1 to 1.5, and D is present in an amount of 0.2-5% by weight of the total composition.
It should be noted that U.S. Pat. No. 4,286,023 relates to strippable outer semiconducting layers. Inner and non-strippable outer semiconducting layers are not disclosed.
It should also be noted that ethylene-vinyl acetate copolymer is the preferred component (A) according to U.S. Pat. No. 4,286,023. If component (A) is selected from C1-C8 alkyl esters of acrylic acid and methacrylic acid, the preferred copolymer is ethylene-ethyl acrylate copolymer.
Although prior art compositions for semiconducting layers in electric cables are satisfactory for many applications, there is always a desire to improve their characteristics and eliminate or reduce any disadvantages they may have.
One disadvantage of EVA conventionally used in semiconducting layers is that at elevated temperatures, such as during compounding of the semiconducting composition, EVA starts to decompose and generate acetic acid at about 150xc2x0 C. At the same time double-bonds are formed in the polymer chain. The acetic acid, which is very corrosive, especially at high temperatures, attacks the processing equipment and leads to an undesired corrosion thereof. To a certain extent this may be counteracted by making the equipment of special, corrosion-resistant materials which, however, are expensive and add to the investment cost for manufacturing the cable. The release of acetic acid is also a negative factor from an environmental point of view. Further, the formation of double-bonds in the polymer chain at the generation of acetic acid may lead to undesired cross-linking and gel formation.
Another disadvantage of EVA as a material for the semiconducting layers of electric cables manifests itself when cross-linking (vulcanising) cables. The cross-linking is usually conducted in an about 100-200 m long vulcanising tube, where cross-linking should take place as rapidly and completely as possible. For conventional cables having semiconducting EVA-containing layers, cross-linking is carried out at a temperature of about 260-300xc2x0 C., preferably 270-285xc2x0 C. A nitrogen-gas pressure of about 8-10 bar is applied in the vulcanising tube and contributes to the preventing of oxidation processes by keeping away the oxygen of the air and to reducing the formation of microcavities, so-called voids, in the polymer layers. As explained above in connection with compounding of EVA, the elevated temperature at the cross-linking of EVA also causes generation of acetic acid and gel formation. The more elevated temperature at the cross-linking step compared to the compounding step results in a correspondingly increased generation of acetic acid and formation of gel. Besides having an obnoxious smell, the acetic acid generated means a loss of VA from the EVA-containing layer and, probably connected therewith, a reduced strippability when making cables with a strippable outer semiconducting EVA-containing layer. Further, the acetic acid released condenses in the vulcanising tube together with other volatile substances and forms a viscous sticky liquid at the bottom of the vulcanising tube. This liquid must be removed from the vulcanising tube as otherwise it tends to adhere to and contaminate the surface of the cable. This implies production stops and lower productivity.
Yet another problem with electric cables is the so-called xe2x80x9cshrink-backxe2x80x9d phenomenon, which manifests itself in particular when standard type EEA or EBA based semiconducting compositions are used as inner semiconductive layers on cables with solid conductors. This problem is related to the fact that the metal conductor of the cable and the polymer coating layers of the cable shrink differently when cooled. After making the cable by extrusion and cross-linking of the polymer layers around the metal conductor as described earlier, the cable is cut into lengths of a suitable dimension and cooled to ambient temperature. Upon cooling the polymer layers of the cable shrink more than the metallic conductor. This shrinking decreases the diameter of the cable coating and also decreases its length along the cable. The last mentioned lengthwise shrinking makes the metallic conductor protrude beyond the cable coating at both ends of the cable (the coating shrinks back from the metallic conductor). The shrink-back of the cable coating also depends on adhesion between the coating, more particularly the inner semiconducting layer, and the metal conductor. The better the adhesion between the inner semiconducting layer and the metal conductor, the smaller the shrink-back, because the increased friction to the conductor inhibits the relaxation of the polymer layer. Standard type ethylene ethyl acrylate (EEA) or ethylene butyl acrylate (EBA), i.e. EEA or EBA with a substantially uniform or random comonomer distribution, both exhibit a relatively high amount of shrink-back compared to EVA due to poor adhesion to the conductor. It should be added that the shrink-back phenomenon is more pronounced for cables with a solid conductor than for cables with stranded conductors due to the smaller area of contact between the metal and polymer in the first-mentioned case.
If one tries to overcome the shrink-back problem of standard type EEA and EBA by increasing the amount of EA and BA in the polymer, respectively, the mechanical characteristics of the EEA and EBA polymers deteriorate to an unacceptable degree. This explains why EEA and EBA have not replaced EVA as polymer for the inner semiconducting layer of electric cables with a solid conductor.
To sum up, EVA is normally used as polymer for semiconducting layers of electric cables, although it has poor thermal stability and decomposes with generation of acetic acid at high temperatures. EEA and EBA are used as polymers for bonded outer semiconducting layers and for inner semiconducting layers only on cables with stranded conductors, due to the above-mentioned shrink-back problem.
The present invention is based on the discovery that the above problems and disadvantages of the prior art are solved or alleviated by using specific, non-uniform or heterogeneous ethylene-alkyl (meth)acrylate copolymers, preferably heterogeneous ethylene copolymers with methyl (meth)-acrylate (M(M)A), ethyl (meth)acrylate (E(M)A), (iso-)propyl (meth)acrylate (P(M)A) or butyl (meth)-acrylate (B(M)A), as the ethylene copolymer in the semiconducting layer.
By the term xe2x80x9cheterogeneousxe2x80x9d used herein is meant a copolymer with the comonomer units distributed non-randomly in the polymer chains as opposed to a homogenous copolymer wherein the comonomer units are distributed randomly in the polymer chains. Usually, homogeneous copolymers are obtained by polymerisation in an autoclave. The term xe2x80x9calkyl (meth)acrylatexe2x80x9d used herein refers to alkyl acrylate as well as alkyl methacrylate. The ethylene-alkyl (meth)acrylate copolymers have high melting points, are highly temperature stable and do not split off any acetic acid when being processed at temperatures that would be critical for an EVA-based composition. In addition, the specific, non-uniform ethylene-alkyl (meth)acrylate copolymers according to the present invention surprisingly lower the amount of carbon black required to make the composition semiconductive, something that confers a distinct economical advantage.
Thus, the present invention provides a semiconducting composition for electric cables which, based on the total weight of the composition, comprises
(a) 50-90% by weight of an ethylene copolymer,
(b) carbon black in an amount at least sufficient to make the composition semiconducting,
(c) 0-8% by weight of a peroxide cross-linking agent,
(d) 0-8% by weight of conventional additives, characterised in that
the ethylene copolymer (a) is a heterogeneous ethylene-alkyl (meth)acrylate copolymer, which besides ethylene moieties comprises 2-10 mole % of an alkyl(meth)acrylate comonomer moiety, chosen among methyl(meth)acrylate, ethyl(meth)acrylate, (iso-)propyl(meth)acrylate and butyl(meth)acrylate, and the melting point of which is at least 95xc2x0 C. and is higher than (108.5xe2x88x921.7xc3x97(mole % of alkyl(meth)acrylate comonomer)) degree centigrade.
The present invention also provides an electric cable including a conductor which, in the order from inside and outwards, is surrounded by an inner semiconducting layer, an insulating layer, and an outer semiconducting layer, at least one of said inner and outer semiconducting layers being derived from a semiconducting composition which, based on the total weight of the composition, comprises
(a) 50-90% by weight of an ethylene copolymer,
(b) carbon black in an amount at least sufficient to make the composition semiconducting,
(c) 0-8% by weight of a peroxide cross-linking agent,
(d) 0-8% by weight of conventional additives, characterised in that
the ethylene copolymer (a) is a heterogeneous ethylene-alkyl (meth)acrylate copolymer, which besides ethylene moieties comprises 2-10 mole % of an alkyl(meth)acrylate copolymer moiety, chosen among methyl(meth)acrylate, ethyl(meth)acrylate, (iso-)propyl(meth)acrylate and butyl(meth)acrylate, and the melting point of which is at least 95xc2x0 C. and is higher than (108.5xc3x971.7xc3x97(mole % of alkyl(meth)acrylate comonomer)) degree centigrade.
The present invention further provides a method of producing a cross-linked semiconducting layer of an electric cable including a conductor which, in the order from inside and outwards, is surrounded by an inner semiconducting layer, an insulating layer, and an outer semiconducting layer, at least one of said inner and outer semiconducting layers being derived from a cross-linkable, semiconducting composition which, based on the total weight of the composition, comprises
(a) 50-90% by weight of an ethylene copolymer,
(b) carbon black in an amount at least sufficient to make the composition semiconducting,
(c) 0.2-8% by weight of a peroxide cross-linking agent,
(d) 0-8% by weight of conventional additives,
characterised in that
the ethylene copolymer (a) is a heterogeneous ethylene-alkyl(meth)acrylate copolymer, which besides ethylene moieties comprises 2-10 mole % of an alkyl(meth)acrylate comonomer moiety, chosen among methyl(meth)acrylate, ethyl(meth)acrylate, (iso-)propyl(meth)acrylate and butyl(meth)acrylate, and the melting point of which is at least 95xc2x0 C. and is higher than (108.5xc3x971.7xc3x97(mole % of alkyl(meth)acrylate comonomer)) degree centigrade, and that the composition is cross-linked at a temperature of 300-400xc2x0 C.
Further characterising features and advantages of the present invention will appear from the following description and the appended claims.